Why normal map baking break at UV seams?
The results are correct. The reason it appears wrong is because the normal map is using tangent space (which is what you usually want) - tangent direction is based on the UV map, so there will usually be a difference in colour where there is a UV seam.

However the colour difference is not an artefact. When used as a normal map (not as a colour map), everything will display correctly:



(That's the low poly model)